"Most of the products on test this month saw another increase in the percentage of spam they missed," said Martijn Grooten at VB UK. "It does suggest that spammers are getting better at finding ways to evade filters -- it may even be the case that mass actions against many spam-sending botnets have forced spammers to look for improved evasion techniques."

The VBSpam test consisted of 131,182 emails sent in real time, 120,763 of which were spam. It ran from June 23 to July 9.

All but three of the 21 tested antispam products scored lower this month than they did in the previous test. "This is a worrying trend that, from the end-user's point of view, undoes a lot of the good work that has been done in reducing the global volume of spam," says Grooten. "When we first reported this trend back in March, many responded by saying that catch rates are still high. While this is true, and we're certainly not losing the war against spam yet, we are losing an important battle."

The VBSpam scoring method takes the spam catch rate, and subtracts five times the false positive rate to arrive at a final score. So if a product catches 98 percent of the spam sent its way, but misidentifies 0.1 percent of legitimate mail as being spam, it receives a score of 97.5 = 98 - (5 * 0.1). VB awards its VBSpam certification to any product that receives a score of 97 or higher.

In addition, the VBSpam+ award is given to any product that catches at least 99.5 percent of all spam, with no false positives.